1. Field Of The Invention
The present invention relates to load cell devices for use with lifting equipment such as cranes and more particularly relates to a tension load cell apparatus that can be used with a pair of spaced apart lifting shackles that are attached respectively to the ends of the load cell and wherein the shackles are modified according to the method of the present invention using insulating bushings and higher load capacity shackle pins. A smaller load cell is provided, in comparison to similar load cells that can be easily transported by a single worker.
2. General Background
Previous load cell designs have relied primarily on two less desirable approaches. One method involves milling padeyes on the ends of an oversized tension member. This results in a high cost machining operation and excessive weight which is often difficult or impossible to manage without a forklift or like motorized lifting device. This approach does not provide a simple surface configuration to mount a protective casing over the strain gages. Another method in common use is to forego fitting a tension load cell to standard shackles, and rely upon various adapters to connect the load cell to the shackle. Often these adapters are threaded assemblies introducing a source of possible failure, which do not inspire confidence in the end user.
Interference from static electricity can build up on the crane boom during a lift. This is the single most important source of erratic or uncertain load readings when using strain gaged load cells in rigging. The crane boom essentially behaves as a giant antenna, picking up electrical noise from generators, arc welders, motors, storm clouds, and rain. This "noise" corrupts the accuracy and repeatability of the load reading.
Taking weights in a construction yard and load testing crane booms seldom require accuracy greater than .+-.1%. However, most commercial load cell manufacturers (not being directly involved in the lifting industry) are convinced that 0.25% or 0.1% or better accuracy is essential in load cell design. Therefore, price schedules usually reflect this philosophy. The manufacturing cost involved in producing greater precision load cells increases exponentially not linearly. For example, to produce a cell with a fundamental accuracy/linearity/repeatability of about 0.5% may be one tenth the cost of the same cell specified to 0.1%.
A strain gage installation must be protected from the elements. Common prior art designs weld a casing around the installation. This method distorts the strain in the tension member, not only because of the weld, but also because of the reinforcing effect of the casing. This does not allow the structural member to behave in pure tension due to applied load.
Most commercial load cell manufacturers are fond of touting their welded casings as a hermetic seal. Hermetic seals may find limited application in severe environmental duty (corrosive chemical immersion, deep sea etc.) but have little real need in the general purpose lifting industry.